U.S. patent number 6,341,661 [Application Number 09/573,520] was granted by the patent office on 2002-01-29 for bow dome sonar.
This patent grant is currently assigned to L3 Communications Corporation. Invention is credited to Ernest Theodore Bick, Merrill E. Fife, Scott A. Hudson.
United States Patent |
6,341,661 |
Bick , et al. |
January 29, 2002 |
Bow dome sonar
Abstract
A compact, modular sonar assembly for use on the bow of a ship
having separate transmitting and receive arrays confined within a
single acoustic housing. The narrow and compact design fits much
better into the hydrodynamically desirable bulbous bow deign than
the typical bow dome sonar design and achieves good performance at
a low cost with reduced size and weight.
Inventors: |
Bick; Ernest Theodore (Newhall,
CA), Fife; Merrill E. (Canyon Country, CA), Hudson; Scott
A. (Stevenson Ranch, CA) |
Assignee: |
L3 Communications Corporation
(Sylmar, CA)
|
Family
ID: |
26894362 |
Appl.
No.: |
09/573,520 |
Filed: |
May 17, 2000 |
Current U.S.
Class: |
181/110; 181/111;
181/140; 181/139; 181/112 |
Current CPC
Class: |
G10K
11/008 (20130101); G01S 7/521 (20130101) |
Current International
Class: |
G01S
7/521 (20060101); G10K 11/00 (20060101); G01V
001/00 () |
Field of
Search: |
;181/110,111,112,120,123,124,125,139,140,141 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hsieh; Shih-Yung
Attorney, Agent or Firm: Roberts & Mercanti, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
Applicant hereby claims the benefit of provisional patent
application No. 60/199,007 filed Apr. 19, 2000 which is
incorporated herein by reference.
Claims
What is claimed is:
1. A sonar assembly comprising:
a) an elongated array of sequentially juxtaposed transmitting
transducer elements stacked within and confined by a pressure
housing, each transducer being independently connected to drive
circuitry;
b) opposite ends of the pressure housing being attached internally
to first and second portions of a frame, which frame is mountable
on a ship;
c) a plurality of staves sequentially positioned around a periphery
of the frame, each stave extending between the first and second
portions of the frame;
d) a series of acoustic receive hydrophones positioned along each
stave, each hydrophone being independently connected to signal
detecting circuitry; and
e) an acoustically transparent housing encapsulating the frame.
2. The sonar assembly of claim 1 wherein the transmitting
transducer elements have a resonant frequency in the range of from
about 1000 HZ to about 10000 Hz.
3. The sonar assembly of claim 1 wherein the transducer elements
comprise flextensional transducers, flexural disk transducers, or
slotted cylinder transducers.
4. The sonar assembly of claim 1 wherein the transducer elements
comprise a stack of high power density electostrictive ceramic
material.
5. The sonar assembly of claim 1 wherein the transducer elements
comprise a stack of high power density magnetostrictive
material.
6. The sonar assembly of claim 1 where each transducer element
comprises a concave or convex flextensional transducer which
comprises a hollow, elliptical shell comprising a pair of concave
or convex side walls meeting at opposing ends; said walls and ends
delineating opposing open sides; a high power density
electostrictive ceramic stack or magnetostrictive stack positioned
in the hollow elliptical shell and extending between the opposing
ends and adapted to exert a force on the opposing ends and strain
the concave or convex side walls when the stack is subjected to
sufficient driving voltage through electrodes bonded to the
stack.
7. The sonar assembly of claim 6 where each transducer element is
attached to a mating transducer element by means of an intermediate
elastomeric seal attached to an open side of each transducer
element, the transducer elements being positioned such that the
electrostrictive or magnetostrictive stack of one transducer
element is parallel to the electrostrictive or magnetostrictive
stack of the mating transducer element; opposite open sides of each
pair of mating transducer elements being closed by an endcap thus
forming a hollow ellipsoid container.
8. The sonar assembly of claim 7 wherein a plurality of the hollow
ellipsoid containers are attached to each other in stacked
container pairs within the pressure housing such that each of the
ceramic stacks are parallel and extend in a direction between the
first and second portions of the frame.
9. The sonar assembly of claim 7 wherein a plurality of the hollow
ellipsoid containers are attached to each other in eight stacked
container pairs within the pressure housing.
10. The sonar assembly of claim 7 wherein each transducer element
is separated from its mating transducer element by a distance of
about 1/4 wavelength at the center of the operating frequency band
of the assembly.
11. The sonar assembly of claim 7 wherein each hollow ellipsoid
container of each pair is separated from its corresponding hollow
ellipsoid container of each pair by a distance of about 1/4
wavelength at the center of the operating frequency band of the
assembly.
12. The sonar assembly of claim 8 wherein each hollow ellipsoid
container of the stack is separated from an adjacent hollow
ellipsoid container of the stack by a distance of about 1/2
wavelength at the center of the operating frequency band of the
assembly.
13. The sonar assembly of claim 6 wherein the high power density
ceramic stack comprises lead magnesium niobate or lead magnesium
niobate-lead titanate.
14. The sonar assembly of claim 1 wherein the hydrophones are
arranged in pairs along each stave.
15. The sonar assembly of claim 1 comprising means for sampling
analog signals received by the hydrophones and generating a digital
signal from the analog signals.
16. The sonar assembly of claim 1 comprising means for sampling
analog signals from pairs of hydrophones along each stave and
generating digital signals corresponding to each pair of analog
signals.
17. The sonar assembly of claim 1 wherein the transmitting
flextensional transducers are substantially identical.
18. The sonar assembly of claim 1 wherein the pressure housing
comprises a cylinder.
19. The sonar assembly of claim 1 wherein the pressure housing is
pressurized internally at from about 40 psi to about 50 psi.
20. The sonar assembly of claim 1 wherein the pressure housing is
filled with a fluid.
21. The sonar assembly of claim 1 wherein the acoustically
transparent housing is pressurized internally at about 15 psi to
about 25 psi.
22. A bow dome sonar assembly for connection to the bow of a ship
comprising:
a) an elongated array of sequentially juxtaposed transmitting
transducer elements stacked within and confined by a pressure
housing, each transducer being independently connected to drive
circuitry;
b) opposite ends of the pressure housing being attached internally
to first and second portions of a frame, which frame is mountable
on a ship;
c) a plurality of staves sequentially positioned around a periphery
of the frame, each stave extending between the first and second
portions of the frame;
d) a series of acoustic receive hydrophones positioned along each
stave, each hydrophone being independently connected to signal
detecting circuitry; and
e) an acoustically transparent housing encapsulating the frame.
23. The sonar assembly of claim 22 wherein the high power density
ceramic stack comprises lead magnesium niobate or lead magnesium
niobate-lead titanate.
24. A process for detecting underwater objects comprising:
a) providing a sonar assembly comprising:
i) an elongated array of sequentially juxtaposed transmitting
transducer elements stacked within and confined by a pressure
housing, each transducer being independently connected to drive
circuitry;
ii) opposite ends of the pressure housing being attached internally
to first and second portions of a frame, which frame is mountable
on a ship;
iii) a plurality of staves sequentially positioned around a
periphery of the frame, each stave extending between the first and
second portions of the frame;
iv) a series of acoustic receive hydrophones positioned along each
stave, each hydrophone being independently connected to signal
detecting circuitry; and
v) an acoustically transparent housing encapsulating the frame;
b) transmitting an acoustic signal from a plurality of transmitting
transducer elements into a fluid medium;
c) receiving a reflected acoustic signal via the acoustic receive
hydrophones;
d) sampling analog signals received by the hydrophones via sampling
means; and
e) generating a digital signal from the analog signals.
25. The process of claim 24 further comprising displaying a
representation of the digital signal via a display.
26. The process for detecting underwater objects of claim 24
wherein analog signals received from pairs of hydrophones are
sampled and digital signals are generated corresponding to each
pair of analog signals.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sonar sensor for the underwater
detection of submarines and other underwater obstacles. More
specifically, the invention relates to a compact, modular sonar for
use on the bow of a ship or submarine utilizing flextensional or
other compact transducers with high power density, and a separate
light-weight receive sensors having good performance, low cost and
reduced size and weight compared to other bow mounted long range
sonar systems.
2. Description of the Related Art
As is well known in the art, sonars on-board ships and submarines
are useful for the detection of submarines and other underwater
obstacles. One particular type of such a sonar is mounted in a
ship's or submarine's bow dome. In general, a bow dome sonar array
is a sonar sensor that is mounted on the hull of a ship or
submarine and protected by an acoustically transparent window or
ellipsoidal dome. See, for example, U.S. Pat. No. 5,400,300 which
generally describes a sonar system mounted underwater on the hull
of a ship. See also U.S. Pat. No. 5,719,824 which generally
describes an underwater domed sonar assembly.
Within the acoustic window of a bow dome sonar assembly are sound
wave projecting and receiving transducers. In the field of sonar,
projecting transducers generally are used to project an acoustic
output signal into a body of water in response to an input
electrical signal. Receiving transducers generally are used to
receive acoustic waves reflected back to the sonar assembly and
generate an output signal in response the reflected acoustic
energy.
Prior art bow dome sonars have been known to use a single
transducer array for performing both the transmit function and the
receive function as described above. A major problem associated
with long range (30-40 nautical miles) sonar assemblies of this
type is the size and weight of the assembly. The transition of the
transducers from the transmit function to the receive function
requires a complex switching network that is housed in the ship or
submarine. This adds to the cost and weight of the sonar. Also,
transporting such enormous assemblies at the bow of a ship
decreases the fuel efficiency and operational speed of the
ship.
It is an object of this invention to provide a bow dome long range
sonar assembly having separate transducer arrays for both the
projecting and receiving functions as an alternative to the dual
function transducers of the prior art. In such an arrangement, each
of the separate arrays are positioned within the acoustic window.
Having separate transmit and receive arrays allows for a lighter
and more compact bow dome sonar assembly than known from the prior
art.
Further, the size of typical prior art long range sonar assemblies
for surface ships are so large that they impact the ships hull
design required to house them. This decreases the ships range and
its operational speed. Also, the extremely heavy (e.g., 30-40
tons), prior art bow dome sonar assemblies require large amounts of
energy to drag them through a body of water. The large size and
heavy weight of this assembly adversely affects the ship's
hydrodynamics and causes increased drag and a decrease in fuel
efficiency.
It is an object of this invention to provide a bow dome sonar
assembly that will give similar high performance as prior art
designs in a much smaller package at considerable savings in weight
and cost. This design, having separate transmitting and receive
arrays, is a narrower and more compact bow dome design than the
prior art. Further, it fits much better into the hydrodynamically
desirable bulbous bow design which is narrower and taller in shape
as compared to a typical sonar bow dome design. This design also
eliminates the need for a complex switching network, and receive
cabinets, and allows the use of imbedded towed array telemetry
because separate transducers are used for the transmit and receive
function. Also, the volumetric design of the receive array forms
its own virtual baffle and discriminates against reflections from
the ship's or submarine's structure.
SUMMARY OF THE INVENTION
The invention provides a sonar assembly comprising:
a) an elongated array of sequentially juxtaposed transmitting
transducer elements stacked within and confined by a pressure
housing, each transducer being independently connected to drive
circuitry;
b) opposite ends of the pressure housing being attached internally
to first and second portions of a frame, which frame is mountable
on a ship or submarine;
c) a plurality of staves sequentially positioned around a periphery
of the frame, each stave extending between the first and second
portions of the frame;
d) a series of acoustic receive hydrophones positioned along each
stave, each hydrophone being independently connected to signal
detecting circuitry; and
e) an acoustically transparent housing encapsulating the frame.
The invention also provides a bow dome sonar assembly for
connection to the bow of a ship or submarine comprising:
a) an elongated array of sequentially juxtaposed transmitting
transducer elements stacked within and confined by a pressure
housing, each transducer being independently connected to drive
circuitry;
b) opposite ends of the pressure housing being attached internally
to first and second portions of a frame, which frame is mountable
on a ship or submarine;
c) a plurality of staves sequentially positioned around a periphery
of the frame, each stave extending between the first and second
portions of the frame;
d) a series of acoustic receive hydrophones positioned along each
stave, each hydrophone being independently connected to signal
detecting circuitry; and
e) an acoustically transparent housing encapsulating the frame.
The invention further provides a process for detecting underwater
objects comprising:
a) providing a sonar assembly comprising:
i) an elongated array of sequentially juxtaposed transmitting
transducer elements stacked within and confined by a pressure
housing, each transducer being independently connected to drive
circuitry;
ii) opposite ends of the pressure housing being attached internally
to first and second portions of a frame, which frame is mountable
on a ship or submarine;
iii) a plurality of staves sequentially positioned around a
periphery of the frame, each stave extending between the first and
second portions of the frame;
iv) a series of acoustic receive hydrophones positioned along each
stave, each hydrophone being independently connected to signal
detecting circuitry; and
v) an acoustically transparent housing encapsulating the frame;
b) transmitting an acoustic signal from a plurality of transmitting
transducer elements into a fluid medium;
c) receiving a reflected acoustic signal via the acoustic receive
hydrophones;
d) sampling analog signals received by the hydrophones via sampling
means; and
e) generating a digital signal from the analog signals.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the bow dome sonar assembly having
separate transmitting and receiving arrays.
FIG. 2 is an elevational view of a section of a stave along with
its hydrophones and dual acoustic modules.
FIG. 3 is an elevational view of one stave.
FIG. 4 is a view of the transmitting transducers in a pressure
housing.
FIG. 5 is an elevational view of two adjacent pairs of transmitting
transducers.
FIG. 6 is a perspective view of two separated transmitting
flextensional transducer elements with endcaps removed.
FIG. 7 is a perspective view of a flextensional transducer
element.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention provides a compact bow dome long range sonar assembly
having separate transmitting and receive transducer arrays
positioned within an acoustic housing. Shown in FIG. 1 is a
perspective view of a bow dome sonar assembly 1 according to the
invention. As shown, the assembly 1 comprises an elongated array 3
of sequentially juxtaposed transmitting transducer elements 2
stacked within and confined by an enclosed, preferably cylindrical
pressure housing 4, which is cut-away in this view. Each transducer
is independently connected to drive circuitry so that their
acoustic projections may be individually controlled. Opposite ends
of the pressure housing 4 are attached internally to top and bottom
portions of a frame 17 which is mountable on a ship's bow 5. Within
the context of this invention, a ship includes a submarine. A
plurality of staves 6 are sequentially positioned around a
periphery of the frame 17 such that each stave extends between a
top of the frame 16 and a bottom of the frame 18. A series of
acoustic receive hydrophones 8 are positioned along each stave 6
wherein each hydrophone 8 is independently connected to signal
detecting circuitry. An acoustically transparent housing 12 then
encapsulates the frame.
There are various types of transmitting transducers known in the
art. Commonly used transducers include flextensional transducers,
flexural disk transducers, slotted cylinder transducers, split bias
transducers and the like. All of these, and potentially others as
well are suitable for application to this invention. Transducers
which are most suitable for use in this invention use high power
density transduction material. As used in this invention, the term
high power density means the combination of the transduction
material and the vibrating unit produces high acoustic intensity
where acoustic intensity is the average rate of flow of energy
through a unit area normal to the transducer radiating surface e.g.
Joules per second per square meter or watts per square meter.
Preferably the transducers of this invention have an acoustic
intensity of about 1200 watts/m.sup.2 or more. See, for example,
U.S. Pat. No. 5,239,518 which describes a high power density
transducer drive material.
U.S. Pat. No. 4,709,361 describes slotted cylinder transducers and
U.S. patent application Ser. No. 09/276,030 describes split bias
transducers. In the most preferred embodiment of the invention, the
transducer elements comprise flextensional transducers. These are
generally described in U.S. Pat. Nos. 5,497,357 and 5,239,518. All
of the above patents and applications incorporated herein by
reference.
In the preferred embodiment, the elongated array 3 comprises
sequentially juxtaposed transmitting transducer elements 2 arranged
in mating pairs. Groupings of two mating pairs then form a
transducer module. Shown in FIG. 7 is a perspective view of a
flextensional transducer element 2 useful for the invention. The
flextensional transducer element comprises a hollow shell 24 having
concave or convex side walls meeting at opposing ends. The walls
and ends delineate opposing open sides and contain a ceramic stack
26 positioned in the hollow shell which extends between the
opposing ends. This high power density ceramic stack 26 is adapted
to exert a force on the opposing ends and strain the concave or
convex side walls when the stack is subjected to sufficient driving
voltage through electrodes bonded to the stack. FIG. 6 shows a pair
of transducer elements in disassembled form. Shown are two open
hollow shells 24, each containing a high power density ceramic
stack 26. Each shell is then covered by an endcap 28. Electrodes 20
(housed within waterproof connectors) which extend through the cap
electrically connect to the ceramic stack. Referring to FIG. 5, in
the preferred embodiment, each individual transducer element 2 is
attached to a mating transducer element 2 by means of an
intermediate elastomeric seal 30 thus forming an attached pair of
transducer elements in the form of a hollow oval cylinder or shell.
The seal 30 is attached to an open side of each transducer element.
Alternatively 30 may be a spacer, such as a metal spacer, between
the shells 24 and the spacer and shells may be encircled by a
sealing cover or boot. Opposite open sides of each pair of mating
transducer elements 2 are closed by an endcap 28 thus forming a
hollow oval container. When two transducer pairs are positioned
together side by side, they form a transducer module as shown. In
the preferred embodiment, eight modules are employed, although the
number may be greater or lesser. Each of the individual transducers
2 are independently connected to drive circuitry by accompanying
electrodes 20 suitable to provide the requisite signals to the
transducer from an on-board ship signal generator, not shown. The
high power density ceramic stacks are positioned such that the
stack of one transducer element is parallel to the stack of the
mating transducer element.
Each transducer element is preferably separated from its mating
transducer element by a distance of about 1/4 wavelength at the
center of the operating frequency band of the sonar assembly. Each
transducer pair of each module is separated from its corresponding
transducer pair of a given module by a distance of about 1/4
wavelength at the center of the operating frequency band of the
sonar assembly. Each module of the stacked array is separated from
an adjacent module of the stacked array by a distance of about 1/2
wavelength at the center of the operating frequency band of the
sonar assembly. These separation distances between transducers
allow for a phase delay and horizontal and vertical steering in the
transmission of an acoustic signal into a fluid medium.
The transducers are preferably constructed to provide a compressive
load on the ceramic which is sufficient to compensate for any
tensile stresses induced by hydrostatic load or dynamic drive
conditions. Many different types of flextensional transducers are
known differing in shell shape and symmetry Low frequency
transducers are preferred. Lower resonant frequencies are achieved
by utilizing larger and/or thinner walled shells generally leading
to larger transducers. The low frequency transducers exhibits low
attenuation of the acoustic signals in sea water and are used when
it is required that transducer have a higher power output.
Preferably the transmitting transducers have a resonant frequency
in the range of from about 1000 HZ to about 10000 Hz. The high
acoustic power outputs and low resonant frequencies increase the
range and hence, the utility of the transducer. Flextensional
transducers are preferred since they have wider bandwidths, are
compact at lower operating frequencies, and higher power handling
capabilities than other types of transducers of comparable size and
weight. One preferred flextensional transducer (the "Class IV
flextensional transducer") includes a cylindrical or rectangular
ceramic driver mounted within and along the major axis of an
elliptically shaped shell.
Each high power density ceramic stack 26 comprises a number of
ceramic plates between which are sandwiched metal electrodes, these
in turn being connected in parallel. Each stack element is flat and
preferably rectangular or circular and may range from about 0.5
inch to about 6 inches in length and width and from about 0.005 to
about 0.5 inch in thickness. The total stack has a length which
fits in the shell 24. Each stack element is attached to the next
stack element as well as to the end elements by a suitable adhesive
such as an epoxy which will not lose its adhesion during transducer
operating conditions. The stack may operate at room temperature,
below room temperature or above room temperature. The preferred
operating temperature to achieve high power over continuous
operation may range from about 10.degree. C. to about 130.degree.
C., more preferably from about 20.degree. C. to about 100.degree.
C. and most preferably from about 20 C. to about 90.degree. C. The
stack can be heated by rod heaters or blanket heaters attached
either directly to the stack elements or to the shells.
The ceramic stacks comprise any suitable ceramic crystalline
material such as electrostrictive, piezoelectric or
magnetostrictive materials. If the ceramic crystal is subjected to
a high direct current voltage during the manufacturing process, the
ceramic crystal becomes permanently polarized and operates in the
piezoelectric region. The electrical signal is then applied to the
ceramic stack to generate mechanical vibrations. As an alternative,
direct current voltage can be temporarily applied to the ceramic
stack during operation to provide polarization of the crystal.
Under these conditions, the operation of the projector is in the
electrostrictive region. After the application of the direct
current voltage is discontinued, the ceramic stack is no longer
polarized. Of the two, piezoelectric ceramic stacks are more
commonly used.
Preferred electrostrictives include lead magnesium niobates (PMN),
lead magnesium niobate-lead titanate (PMN-PT), lead magnesium
niobate-lead titanate-barium titanate (PMN-PT-BA), lead zirconate
niobate (PZN), lead zirconate niobate-barium titanate (PZN-BA) and
Pb.sub.1-x.sup.2+ La.sub.x.sup.3+ (Zr.sub.y Ti.sub.z).sub.1-x/4
O.sub.3, (PLZT). Preferred piezoelectrics include lead zirconate
titanate (PZT), barium titanate (BT) and NbLiO.sub.3. Preferred are
lead magnesium niobates (PMN), preferably lead magnesium
niobate-lead titanate (PMN-PT) as is well known in the art.
Preferably the lead magnesium niobate has a Curie temperature Tm
approximately equal to the operating temperature of the
electro-acoustic transducer. PMN-PT materials are particularly
attractive in high power projector applications because they offer
figure of merit improvements of up to 11 dB compared with
conventional PZT. This increase can be used to produce higher peak
source levels without significant impact to system size/weight, or
it can be used to achieve comparable system performance in smaller,
lighter weight arrays. The term PMN-PT is used to describe a family
of ceramics whose electrostrictive properties vary widely. The
ratio of Lead Titanate (PT) (and other materials) to PMN affects
both the material performance (dielectric, loss tangent, coupling,
etc.) and the temperature at which these properties are maximized
(Tm). A Tm=85.degree. C. PMN material for the transducer is
preferred. The material has excellent electrostrictive properties
but also exhibits other mechanical and electrical properties which
make it a more usable material than other PMN ceramics. PMN-PT
compositions offer dramatically higher strain rates than PZT
ceramics and thus higher acoustic source levels when used to drive
a transducer. Other useful ceramic materials non-exclusively
include PMNRT (Tm=25.degree. C.), PMN-10/3 (Tm=85.degree. C.),
PMNHT (Tm=85.degree. C.) and PZT8 (Tm=25.degree. C.).
Magnetostrictive materials include nickel alloys and
rare-earth-iron alloys. A preferred magnetostrictive material is
terbium-dysprosium-iron alloy known under the trade name
Terfenol.
Transmitting transducers, or projectors, generally include a
mechanically driven member such as a piston, shell, or cylinder and
a driver. The driver is responsive to electrical energy and
converts such energy into mechanical energy to drive the
mechanically driven member. The driven member converts the
mechanical energy into acoustic waves which propagate in the body
of water. Most acoustic transducers have driver elements which use
materials having either magnetostrictive or piezoelectric
properties. Magnetostrictive materials change dimension in the
presence of an applied magnetic field, whereas piezoelectric
materials undergo mechanical deformation in the presence of an
electrical field. A common piezoelectric driver is the ceramic
stacked driver which is made up of individual ceramic elements
which are stacked with alternating polarities. In this stacking
arrangement, the ceramic stack is longitudinally polarized.
Electrical drive is applied to the elements of the ceramic stack
and in response, each element expands and contracts in the
longitudinal direction. The individual element displacements
accumulate to provide a net displacement of the stack. Pre-stress
is applied to the stack by compressing the shell along its minor
axis, thereby extending the major axis dimension allowing a
slightly oversized ceramic stack driver to be placed along the
major axis. Releasing the compressive force applied to the
elliptical shell places the stack in compression. With this
configuration, the elliptical shell acts as a mechanical impedance
transformer between the driving element and the medium, such as a
body of water, in which the transducer is disposed. In some
flextensional configurations, the ceramic stack is made in two
parts and the pair of ceramic drivers are separated with a support
structure, having an I-beam frame disposed across the minor axis of
the elliptical shell for providing a mounting surface for endplates
at each end of the shell. The endplates 28 seal the transducer
stack and protect the inner components from the outside medium. As
used in this manner, the stack support structure provides a thermal
path for dissipating heat generated in the ceramic stack driver.
This heat sinking feature can be very important in high power
applications. The dynamic excitation of the ceramic stack driver
causes the stack to expand and contract. A small velocity imparted
at the ends of the ceramic stack is converted to a much larger
velocity at the major faces of the elliptical shell resulting in
the generation of an acoustic field within a medium in which the
transducer is disposed. It is generally desired for good
electro-acoustic efficiency that contact is made to the drive
points of the shell only by the ceramic stack assembly. The support
structure and end plates are generally physically isolated from the
shell. In this arrangement, the flextensional transducer is said to
be "air-backed", that is, air is disposed in contact with the shell
and the support structure.
The transducer shells may be composed of any suitable material such
as steel, aluminum, fiberglass or suitable polymeric material. Wall
thickness can be easily determined by those skilled in the art,
however, wall thickness in the range of from about 0.25 inch to
about 3 inches are useful. Each shell may have any convenient
length and width, such as a height of from about 7 inches to about
4 feet and a width of from about 1.5 inches to about 2 feet.
The transducer assembly with high power density electostrictive
ceramic preferably has biasing means for providing a first
electrical signal to polarize the ceramic stacks. Means are also
provided for applying an alternating current driving signal to each
of the transducers. This generates acoustically in-phase output
signals from each transducer. The AC drive signals are separately
applied across each transducer. The transducers are designed to
present similar electrical impedances to the drive voltage.
Impedances of the two may be the same or different, however
variations of as much as 30% are tolerable. In the preferred
transducer assembly, the transducer pairs preferably have the same
resonant frequencies.
The array 3 is stacked within and confined by a generally
cylindrical, acoustically transparent pressure housing 4. This
pressure housing prevents cavitation during operation of the sonar
assembly. The housing 4 is attached internally at opposite ends to
top 16 and a bottom 18 portions of a U-shaped frame 17. This frame
is mountable on a ship's hull 5, as shown. Shown in FIG. 4 is a
sectional view of the pressure housing 4 showing the array of
transducers 3 it encapsulates. The array of transducers 3 comprises
a set of preferably substantially identical transducers 2 attached
by a suitable support 9.
The housing 4 of the assembly 1 as well as support 9 may be
composed either of a metal or polymeric material as could be
determined by one skilled in the art. Additionally, the housing 4
is filled with a fluid such as seawater or any other fluid
appropriate for purposes within the scope of this invention. It is
well known in the art that operation of sonar devices can cause
cavitation due to the negative pressures created by the generated
sound field. These negative pressures cause the pressure of the
water to drop below the vapor pressure, thereby allowing the water
to vaporize. To combat this problem, the pressure housing 4 is
maintained at a pressure ranging from about 40 to about 50 psi.
Referring again to FIG. 1, sequentially positioned around a
periphery of the frame are a plurality of staves 6. These staves 6
extend between the first top portion 16 and a second bottom portion
18 of the frame 17. Positioned along each stave 6 is a series of
pairs of acoustic receive hydrophones 8, as can be seen in FIGS. 2
and 3. The staves are long, narrow strips of metal, polymer or
other material used to support the hydrophones and dual acoustic
modules. The receive array may comprise from about 20 to about 80
staves, more preferably from about 30 to about 50 staves. FIG. 3
shows a full size elevational view of a single stave 6. Typically
they are from about 6 feet to about 8 feet long, however longer or
shorter staves are within the contemplation of the invention. The
hydrophones 8 are commonly used receivers in towed array sonar
systems. Each of these hydrophones 8 are independently connected to
signal detecting circuitry and are used to intercept reflected
acoustic signals sent back to the sonar assembly. Typically
hydrophones are designed to operate over broad frequency ranges and
are generally small in size relative to the wavelength of the
highest intended operating frequency. Also optionally positioned on
the staves to facilitate reliable telemetry are dual acoustic
modules 10 which sample received analog signals from each
hydrophone or pair of hydrophones.
Surrounding the frame 17 is an acoustically transparent window or
dome 12. This acoustic window 12, as seen in FIG. 1, encapsulates
the frame along with the transmit and receive arrays to form an
enclosed, pressurized chamber. This pressurized chamber further
protects the assembly 1 from cavitation during operation of the
system. The chamber formed by the acoustic window 12 is also filled
with a fluid and is maintained at a pressure ranging from about 15
psi to about 25 psi. Suitable fluids include seawater or any other
fluid appropriate for purposes within the scope of this invention.
Such acoustically transparent domes can have any conventional shape
such as ellipsoidal, circular and the like. Alternately, the dome
can simply conform to a curvilinear portion of a vessel hull 5 and
thereby resemble a window of a building or other structure. The
particular physical form taken by such an acoustic window will be a
function of the particular acoustic wave form
transmission/reception function to be provided by the acoustic
transmitter or receiver equipment positioned behind the dome or
within an enclosure at least partially defined by the dome. The
window may be formed of a suitable or conventional structural
material. This material can be reinforced or unreinforced
thermosetting plastic or reinforced or unreinforced thermoplastic.
Alternately it can be formed from a low density, high modulus metal
or metal alloy, or from carbon composites. A more thorough
description of the properties of suitable acoustic window can be
found in U.S. Pat. No. 4,997,705, incorporated herein by
reference.
In use, an alternating current is sent from a power source on the
ship to electrodes 20 on the projecting transducer elements 2.
Application of this electrical signal causes the high power density
ceramic elements to expand and contract in such a manner to produce
a vibration along the length of each ceramic stack. This vibration
is transmitted to the cylindrical housing 4 and causes a vibration
along the minor axis of the shell. This vibration generates
acoustic waves that are sent past the housing 4 and window 12 and
into the sea water.
Once a projected acoustic wave reaches its target, it is reflected
back to the sonar assembly where it is received by the receive
array. The receive array is configured to allow the assembly to
receive acoustic data from an expansive area in a cardioid vertical
and horizontal pattern, eliminating wave reflections from the hull
of the ship and from the transmitting array, thus minimizing the
noise from which the assembly can receive data. The hydrophones 8
receive the reflected acoustic waves as analog signals. Thereafter,
the acoustic modules 10 sample the received analog signals from
each hydrophone or pair of hydrophones and generating a digital
signal from the analog signals. The sampled data is converted to a
digital signal and transmitted inside the ship to a processor unit
via fiber optic cables.
This enables the ship crew to detect and localize the target which
reflected sound to the ship. The described receive array can also
be used in a passive mode wherein it is not excited by the active
array but receives acoustic signals emitted by submarine or other
targets.
The invention also provides a process for detecting underwater
objects comprising first providing the above described sonar
assembly comprising, then transmitting an acoustic signal from a
plurality of the transducer elements into a fluid medium, receiving
a reflected acoustic signal via the acoustic receive hydrophones,
sampling analog signals received by the hydrophones or hydrophone
pairs via sampling means and generating a digital signal from the
analog signals. A representation of that digital signal is then
displayed via any suitable display means such as a cathode ray
tube.
While the present invention has been particularly shown and
described with reference to preferred embodiments, it will be
readily appreciated by those of ordinary skill in the art that
various changes and modifications may be made without departing
from the spirit and scope of the invention. It is intended that the
claims be interpreted to cover the disclosed embodiment, those
alternatives which have been discussed above and all equivalents
thereto.
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